The present invention relates to a solid-state image sensing device and a cellphone having an image processing function.
Generally, solid-state image sensing devices are roughly classified into a CCD sensor and CMOS sensor. The CMOS sensor in which signal charge is transferred by a signal line is superior in power consumption to the CCD sensor in which two types of pulses are applied to a transfer electrode to transfer signal charge in a semiconductor layer. The conventional CMOS sensor type solid-state image sensing device will be described below.
In the CMOS sensor type solid-state image sensing device, unit pixels are arranged in a matrix in an image sensing region. Each unit pixel includes a photoelectric converter (photodiode) which generates signal charge by photoelectric conversion, a read transistor which reads out this signal charge, a reset transistor which removes the signal charge, an amplifying transistor which amplifies the signal charge and outputs a pixel signal, and an address transistor which selects an address in which the photodiode is present.
The photodiode is formed by forming a P-well (P-type semiconductor region) in a semiconductor substrate, forming a resist film so as to cover a portion except for a prospective photodiode region, ion-implanting an impurity such as phosphorus by using the resist film as a mask, and annealing the resultant structure after the resist film is peeled.
Also, each of the reset transistor, amplifying transistor, selection transistor, and read transistor is formed by depositing a polysilicon film, and patterning this polysilicon film into a desired shape by, e.g., reactive ion etching (to be referred to as RIE hereinafter).
Unfortunately, the following problems arise as the micropatterning of elements advances, the number of pixels increases, and the level of functions rises.    (1) If the photodiode is irradiated with intense light and becomes unable to hold generated signal charge any more, this signal charge leaks into an adjacent photodiode. That is, a so-called blooming phenomenon cannot be prevented.    (2) When an electronic shutter function is added, the signal charge of the photodiode must be removed, i.e., a so-called reset operation must be performed. In the conventional device, this signal charge is removed via the reset transistor and read transistor. However, this method has the problem that it is difficult to control the operations of the reset transistor and read transistor present in a path when the signal charge is to be removed. Especially when the transistor operation control is complicated as the number of pixels increases, the operation margin of the whole device is adversely affected.
A method which solves the problem (1) described above is proposed in reference 1 (to be presented later). In this method, a photodiode is formed by ion-implanting phosphorus (P+) a plurality of number of times, along the direction of depth of a semiconductor substrate.
Also, in reference 2 (to be presented later), the capacity by which a photodiode can store signal charge is increased by forming the photodiode in a deep position of a semiconductor substrate, thereby suppressing or reducing blooming.
Furthermore, the CCD sensor generally uses an N-type substrate as a semiconductor substrate, and signal charge overflowing from a photodiode is discarded to the substrate by applying a voltage of about 15 V to the substrate. That is, a so-called vertical overflow drain structure is proposed and put on the market.
Unfortunately, none of these methods can well suppress blooming, and can solve the problem pertaining to an electronic shutter described in (2) above.
The references disclosing the conventional solid-state image sensing devices are as follows.
Reference 1: Japanese Patent Laid-Open No. 11-74499
Reference 2: Japanese Patent Laid-Open No. 10-257394